The highly demanded embedded system such as 8051/80390 based 8-bit single-chip microcontroller usually features either ROM (Read-Only-Memory), Flash or EEPROM (Electrically-Erasable-Read-Only-Memory) memory as program memory to store the Boot Codes. The Boot Codes is a firmware with predefined function to perform microcontroller general initialization, power-on self-test, some application specific tasks or libraries, or tasks for application code upgrade. In contrast to Boot Codes, the application specific software program codes are usually referred as Run-time Codes.
There are two widely used basic approaches to distribute of ROM, Flash or EEPROM memory for Boot Code storage. The first approach is to completely use internal Boot ROM or a combination of internal Boot ROM and on-chip or off-chip Flash/EEPROM memory. The requirement for this approach is that the software or firmware program in Boot Codes has to be fully or partially stabled and finalized (partially stable and finalized will have that portion to be in ROM and still-in-developing-stage portion in Flash/EEPROM memory). The advantage of this approach is to utilize the high speed access to internal ROM. The disadvantage is a long development cycle and its associated cost to achieve a fully tested and verified software or firmware for the Boot codes. Furthermore, the fixed mask of internal ROM restricts the flexibility to make field upgrade for feature enhancement or bug fix.
The second approach is to use combination of on-chip or off-chip Flash/EEPROM memory without internal Boot ROM. It is not required for the approach that the software or firmware program in Boot Codes has to be fully or partially stabled and finalized. The advantage of this approach is that it is easy to customize or perform field upgrade on final application systems for feature enhancement or bug fix for the software or firmware program in Boot Code. The disadvantage is a relatively slower speed access time to memory in boot-up process. Furthermore, it usually requires at least two banks of Flash or EEPROM memory to support In Application Programming.